Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
If you’ve been watching the eagles, their eggs and their new chick at EagleCam, you may have asked yourself this question: “If the feathers are there to insulate the eagles and prevent heat loss, how can a feathered bird keep its eggs and brood warm?”
The answer is that the eagle isn’t feathered at the part of its body that contacts the eggs and chicks. This involves a nice adaptation called the brood patch. The Stanford University bird webpage says this:
One of the main functions of the feathers is to insulate the bird — to prevent its body heat from being dissipated through the skin surface. Most birds have “solved” the dilemma posed by the need to both transfer and preserve heat by evolving “brood patches.” These are areas of skin on the belly that lose their feathers toward the end of the egg-laying period. In most birds the feathers are shed automatically, but geese and ducks pluck their brood patch and use the plucked feathers to make an insulating lining for their nests. The brood patch also develops a supplemental set of vessels that bring hot blood close to the surface of the skin. When birds return to the nest to resume incubating, they go through characteristic settling movements in order to bring the brood patch into contact with the eggs. In precocial birds, after the chicks have hatched the insulating feathers grow back. In passerines, and presumably other altricial birds, the regrowth of the feathers is delayed, and the patches remain functional through early brooding. Then they gradually disappear, restoring the adult’s thermoregulatory integrity about the time the young are fledged.
The placement of brood patches differs among groups of birds. There may be a single brood patch in the middle of the belly, as in hawks, pigeons, and most songbirds. Shorebirds, auks, and skuas have one on each side, and gulls and game birds combine these two patterns by having three brood patches. Pelicans, boobies, and gannets have none at all. They cradle the eggs in their webbed feet, cover them with the abdomen, and apparently warm them from both above and below.
When just one parent incubates, it alone develops a brood patch. If both parents incubate, both may grow brood patches, or one may cover the eggs without a patch, warming it less efficiently, but at least retarding heat and water loss from the egg.
And here’s a photo showing the position of the eagles’ nest and the webcam:
Be sure to check out the Hummingbirdcam too (you have to watch a brief commercial first). The mother bird (an Allen’s hummingbird) is constantly on and off the nest, frequently feeding her two young. It’s delightful—one of the best animal webcams ever!
Among the first phenomena to be interpreted in a Darwinian manner after the publication of the Origin of Species was adaptive coloration, most famously Batesian mimicry (wherein a palatable organism mimics a noxious organism); Jerry has recently posted on mimicry in insects and in birds. Matthew has brought to our attention a paper by J.M. Kamila and B.J. Bradley, in press in the Journal of Zoology, on another aspect of adaptive coloration: obliterative, or countershading, and in particular how it applies to primates. The Capuchin below is not countershaded.
Capuchin monkey, Guanacaste, Costa Rica, by David M. Jensen, from Wikipedia
Countershading, which is familiar to fishermen and military planespotters, consists of having the illuminated surface of an object darkened, and the unilluminated surface lightened, so as to “counteract the effect of shade and light”, producing “upon a rounded surface the illusionary appearance of flatness” (Cott, 1957:36). As such, it is one of the chief methods by which animals (as well as war planes, at least old ones) achieve concealment, and is very common. Kamila and Bradley, in their paper, ask: If primates spend a lot of time standing up on two legs (like we do), are they less likely to exhibit dorso-ventral countershading? Intuitively, it seems entirely plausible, and, after measuring the reflectance of the front and back of skins of 113 species of primates, they find that, indeed, the more bipedal a primate is, the less strongly it is countershaded. So now we know why our backs and chests/bellies are about the same color– we’re too bipedal!
My experience is that monkeys in trees are hard to see, regardless of whether they are countershaded. The three common Costa Rican species shown in the pictures here, all of which I know in the wild, are hard to spot, even though they are not countershaded. The large white scrotum of the male mantled howler, below, known as “huevos”, do make the males somewhat more conspicuous, but this is almost certainly a sexually selected feature.
Mantled howler, Prov. Alajuela, Costa Rica, by Tim Ross, from Wikipedia.
The fourth Costa Rican monkey species (not pictured here), the squirrel monkey, is countershaded.
[Jerry’s note: I’ve added the picture below, which shows the countershading of a squirrel monkey: it’s darker on the illuminated dorsal (back) side and lighter on the ventral (belly) side:]
Of about 30 species of monkeys in the Guianas, Venezuela, and Colombia, many of which are very strikingly patterned, only a handful might be considered countershaded (Eisenberg, 1989). Perhaps not surprisingly, Kamila and Bradley found that the effect of bipedal tendencies, while significant, was small. They did find that body size made a difference (bigger, less predation-prone primates are less countershaded), but that group size does not (although it was almost significant). Overall, the factors they considered explained only 14% of the variation in countershading in primates.
Spider monkey, Golfo Dulce, Costa Rica, by Steven G. Johnson, from Wikipedia.
Somewhat surprisingly, adaptive coloration was very controversial (critics considering resemblances of mimics and models, and the concealing effects of color patterns, to be coincidental) in Darwin’s time, and continued to be so for decades afterwards. It was not until 1940, that Hugh Cott, one of the 20th century’s most influential herpetologists, put the controversy to rest in his classic Adaptive Coloration in Animals. We’ll conclude with some video, taken by my wife, of monkeys leaping from tree to tree near Tortuguero, Costa Rica. It was a typical lowland Costa Rican day, quite warm, which enables me to label this video as “hot monkey action” (let’s see how many hits that phrase brings in!)
This finding was published in 1989 by Erick Greene (reference and link below), who sent me these photos, but it’s such an astonishing case of mimicry that I showed it to my students last week.
It involves the caterpillar Nemoria arizonaria, the juvenile stage of a moth that lives in Arizona, New Mexico, northern Mexico, Texas and California. It has two generations per year, one in the late winter/early spring and the other in the summer. In both cases the caterpillars, after hatching, live on oak trees and eat parts of them.
If the caterpillars hatch in the winter or early spring, they feed on oak “catkins” (flowers), and, sure enough, their bodies take on the appearance of a catkin, almost certainly to hide them from visual predators like birds. Here’s a “catkin morph” (to the right) next to some real catkins.
When the summer brood hatches, however, the catkins are long gone, and the caterpillars feed on the only food available: oak leaves. This generation looks not like flowers, but like oak twigs:
It’s camouflage again, but a different type. And it’s obviously adaptive, if you have several broods per year, to evolve an appearance that matches the environment in which you hatch. There are also differences not just in appearance, but in their heads and jaws: catkin morphs have smaller jaws suitable for eating the pollen grains, while twig morphs have larger mouthparts and jaws to nom the tougher leaves. Finally, they differ in their behavior: if you put the catkin morph on a twig, it moves back to the flowers, but the twig morph does the opposite.
The interesting thing about these two morphs is that they are genetically identical: a caterpillar of this species has genes that can make it look either like an oak flower, or like an oak twig. Within its genome are two distinct developmental programs coding for its appearance, and which program is activated depends on the season (this temporally varying appearance of a single species is called a developmental polymorphism or a polyphenism). How does the caterpillar know which set of genes to turn on, and when?
The two obvious environmental cues are photoperiod (which differs between winter/spring and summer) and diet. Greene captured moths in the field and reared them on different diet and photoperiod regimes. It turned out that the only factor affecting appearance was diet: caterpillars reared on catkin diets assumed the catkin appearance; those raised on leaves turned into twig morphs. Greene hypothesized that the critical chemical difference involved tannins (polyphenols), which are high in leaves and low in catkins. Sure enough, caterpillars raised on artificial diets supplemented with polyphenols developed into twig morphs, even when they were also fed catkins.
The evolutionary advantage of producing two broods per year is obvious. Although catkins seem to be a superior diet, they’re available only once a year during the short flowering period. Any catkin morph that developed into a moth who was also able to produce twig morphs would leave many more copies of its genes than would a moth constrained to reproduce only once per year.
The precise evolutionary sequence of change, however, is unknown, since all we have is the endproducts. Developmental polymorphisms are not unique to this species—they’re also found in aphids, rotifers, water striders and, of course, the social insects, where every female has the genes for becoming either a queen or a worker.
Despite our ignorance of the evolutionary path, the precision of the mimicry (to use a Stangroomism, look at them) tells us that when the proper mutations are available, natural selection can make an animal look almost identical to its background. In this case we know the “targets” of selection: the appearance of a flower and a twig. And in both cases natural selection gets it spot on. This precision also tells us that the predators—certainly birds—are sharp sighted. If they couldn’t see all that well, there would be no selective advantage to such a precise resemblance. But we all know that birds have keen sight!
By the way, both forms of the caterpillar turn into this lovely geometrid moth, which itself seems to be a leaf mimic:
Chimps and other primates may show a rudimentary form of morality, but it’s clear that by and large nature is pretty heartless—as you’d expect given the character of natural selection. Nevertheless, who hasn’t felt like reproving the cheetah who brings down, on film, a cute young Thompson’s gazelle?
In class this week, my students were saddened and dismayed by another of nature’s nefarious wonders, the cuckoo—in particular, the common cuckoo (Cuculus canorus). Here’s one:
The cuckoo has hit on the mother of all adaptations, a phenomenon called brood parasitism. Instead of spending huge amounts of time building a nest, brooding eggs, and feeding the voracious young until they fledge, the female cuckoo recruits an individual of another bird species to do all the work for her—a full-time babysitter. That way, the cuckoo can use its time and resources maximizing its reproductive effort without the enormous expense of childcare.
A female cuckoo simply lays one egg in the nest of another species, say a reed warbler or a dunnock, where there is already a clutch of eggs. The cuckoo’s egg then nestles inconspicuously among the others. Inconspicuously? Yes, for the female cuckoo lays an egg that mimics pretty well the other eggs in the nest, so the foster mother can’t easily detect the intrusion.
The curious thing is that within the common cuckoo species, each female lays an egg precisely patterned and color to mimic the eggs in the nest she will parasitize. And here’s the kicker: within that cuckoo species there are seven different types of females, each laying a different type off egg and each parasitizing only the nests of species producing similarly-colored eggs. The different “types” of cuckoos that lay different eggs are called gentes. The gentes are not different species of cuckoos—they are all members of the same species, but with different types of genes that make different types of eggs. This is an example of a genetic polymorphism (from the Greek meaning “different forms”). Polymorphisms are not rare in animals and plants: our own species has them, including eye color variants and whether your earwax is wet or dry (a trait based on a single gene).
Here are four of the different types of eggs laid by four cuckoo gentes. The species that is parasitized is on the left, the mimetic cuckoo egg on the right. Again, each female lays only one type of egg her whole life.
Here are some actual nests, each containing a single cuckoo egg, indicated by the arrow. Note that the mimicry is very good but not perfect—a human observer (but not the bird) can pick out slight differences, and the cuckoo egg is often larger:
What happens next is sad, but a remarkable adaptation showing nature in all its red toothiness and clawdom. The cuckoo chick hatches and proceeds to destroy all its competitors—the other eggs and chicks—leaving it the sole recipient of foster care (click on the “Watch on YouTube” line).
As Attenborough notes in the video, brood parasitism has evolved in many bird species, including the “cuckoo duck” Heteronetta atricapilla.
The European cuckoo’s adaptive habit raises lots of questions. Why is the foster parent fooled by the mimetic eggs, but can’t seem to recognize a cuckoo chick that is so different from its own? In some species of brood parasites, like indigobirds, the foster mother can recognize foreign chicks, and so the parasite babies have evolved calls and “mouth gape” patterns (coloration on the babies’ mouths that induce feeding by the mother) that mimic those of the non-foster young. Another explanation is that the foster mother’s drive to feed whatever chick it sees in its nest outweighs everything else: that is, there’s simply no genetic variation for the foster mother to respond to an alien-looking chick. While genetic variation is pervasive in nature, it’s not always around when it’s “needed.” Every case of a parasite or predator victimizing another species, for example, represents an absence (perhaps temporary) of genetic variation in the victim to fully overcome the challenge.
The biggest mystery, though, is how the polymorphism for color pattern is maintained. How, for example, does a female “know” where to lay its egg? If it had the genes for producing eggs that mimic reed warblers, and laid its egg in a dunnock nest, that egg would be summarily ejected and its genes would not be passed on.
This problem is overcome by imprinting: a female imprints on the song and appearance of its foster mother, so when it comes time for a cuckoo to lay its own egg, it goes right back to a nest harboring a female on which it’s imprinted.
But what about mating? Males, after all, also carry genes for egg color and pattern—they just don’t express them. (If you’re a male human, you carry genes for making breasts and vaginas, but don’t express those either.) But if a female mates with a male carrying egg-pattern genes different from hers, wouldn’t the eggs that their daughter produce be intermediate, and therefore unable to pass the test of mimicry?
One possible answer is that a female knows to mate only with those males carrying similar egg-pattern genes. But this isn’t the case. First of all, there’s no way a female can detect a male’s genetic endowment for egg pattern. But more important, research has shown that female cuckoos mate randomly—they don’t know or care what a male’s “egg genes” are. So how does the pattern fidelity work?
We’re not sure, but it may involve the birds’ sex chromosomes. In birds, unlike mammals, it is the female who has two different sex chromosomes, called the Z and the W. In humans males are XY and females are XX, but in cuckoos and other birds (and butterflies), females are ZW and males are ZZ. A ZW female produces ZW daughters, so the W chromosome, and the genes it carries, are transmitted matrilinearly. The male makes no genetic contribution to this chromosome.
This, then, is a possible solution to the egg-color polymorphism. If the genes for a specific egg color and shape are carried only on the W chromosome, then in the offspring those genes will not be mixed with any genes from the father. This will enforce a fidelity of egg type between female and daughter, regardless of who the female mates with. That, combined with the tendency of female cuckoos to imprint, explains how the single common cuckoo species can harbor several types of females, each laying a different mimetic egg, and with very few “mistakes.”
The genetic studies verifying the location of egg-mimic genes on the W chromosome have yet to be done: you can imagine how hard it would be to cross different cuckoos in the lab or aviary, for that would also involve providing foster parents! This is a project for a bright and energetic graduate student.
And, of course, this whole story tells you where the word “cuckold” came from.
This week I lectured on mimicry to my evolution class, a course required for all biology majors. The lecture is a Powerpoint presentation full of amazing mimics, and is designed to show how some evolutionary principles (directional selection, frequency-dependent selection, mutualism, kin selection, and so on) play out in the making of remarkable adaptations.
Mimicry, a phenomenon discovered in the late 19th century, was also important in buttressing Darwin’s theory of natural selection, for some cases admitted of no other explanation. Too, many ideas about how mimicry evolved are testable ones: for example, does conspicuous warning coloration in bad-tasting butterflies—coloration that is learned and avoided by bird predators—also protect the similarly colored but perfectly edible species of mimic butterflies? This shows that evolutionary biology is not just a concatenation of plausible-sounding stories, but yields hypothesis that can be tested in the field or lab.
One of the most remarkable examples of mimicry occurs in some North American freshwater mussels in the genus Lampsilis. Their young go through a parasitic stage, in which they must attach to the gills of fish and suck their blood before later dropping off and resuming a normal mussel-ish life on the stream bottom. But how can a sessile adult mussel get its young into the gills of a fish? The answer involves evolutionary modification of the mussel’s brood pouch—which contains its young—so that it attracts predatory fish. (The brood pouch is simply an outgrowth of the mussel’s mantle.)
But I’ll let the video tell you the story:
Note that, as the video states, the mussel can’t see the fish it’s parasitizing. In this case natural selection is literally blind. Those mutations in the mussel that make its brood pouch look more fishlike will give it a reproductive advantage over its confrères, even if it can’t see the fish it’s deceiving. Note as well that selection has “acted” (I’m anthropomorphizing here: selection doesn’t really “act”, for it’s not an external force but a process of gene sorting) not just on the appearance of the mussel, but on its behavior. It has genes that make it wiggle its brood pouch in a fishlike manner.
See how precise the mimicry is: the brood pouch has fake eyespots, fake fins, and even, in some mussels, a fake mouth that opens and closes. Mimicry is one case in which we know what the evolutionary target, or “optimum,” is, and we can see how close selection can take a species to that target. As in many other cases of mimicry, selection gets it pretty spot on.
If you’re into ants—and who isn’t?—you can’t do better than follow biologist Alex Wild’s excellent blog Myrmecos (the study of ants is called “myrmecology”). It’s one of the best taxon-specific blogs around.
Alex doesn’t like to deal with creationists, but made an exception when Intelligent Design (ID) advocate William Dembski started making pronouncements on ants. Noting that ants tend to take the shortest path between colony entrances (they also do this when travelling between a colony entrance and a food source), Dembski, writing on February 18 at the ID site Uncommon Descent, pronounced this feat inexplicable by natural selection (ergo Jesus):
Now here’s an interesting twist: Colonies of ants, when they make tracks from one colony to another minimize path-length and thereby also solve the Steiner Problem (see “Ants Build Cheapest Network“). So what does this mean in evolutionary terms? In ID terms, there’s no problem — ants were designed with various capacities, and this either happens to be one of them or is one acquired through other programmed/designed capacities. On Darwinian evolutionary grounds, however, one would have to say something like the following: ants are the result of a Darwinian evolutionary process that programmed the ants with, presumably, a genetic algorithm that enables them, when put in separate colonies, to trace out paths that resolve the Steiner Problem. In other words, evolution, by some weird self-similarity, embedded an evolutionary program into the neurophysiology of the ants that enables them to solve the Steiner problem (which, presumably, gives these ants a selective advantage).
I trust good Darwinists will take this in without skipping a beat, mumbling something like “evolution sure is amazing” or “natural selection is cleverer than us.” Dispassionate minds might wonder if something deeper is at stake here.
(“Something deeper,” of course, means Jesus.)
Well, if Dembski had bothered to learn anything about ant trails (and this takes only a few minutes of Googling), he would have realized that embedded in the ants’ tiny brains is not an evolutionary algorithm for solving the Steiner problem, but a simple rule combined with a fact of chemistry: ants follow their own pheromone trails, and those pheromones are volatile. As Wild explains, ants start out making circuitous paths, but more pheromone evaporates from the longer ones because ants take longer to traverse them while laying down their own scent. The result is that the shortest paths wind up marked with the most pheromone, and ants follow the strongest scents.
Wild shows a nice simulation video on his site, demonstrating how, given these simple assumptions, ants wind up taking the shortest trails.
Before we say that evolution can’t explain a behavior, it behooves us to learn as much as we can about that behavior. Dembski didn’t learn jack. And we shouldn’t underestimate the capacity of insect brains to store complicated information, or of evolutionists to decipher how that capacity evolved. A good example is the waggle dance of honeybees.
Alex is actually pretty soft on Dembski, for after pwning him Wild says,
In Dembski’s defense, his error is a common one. Ant societies share enough superficial similarities to human ones that the tendency to anthropomorphize is strong. It is too easy to assume ants solve complex problems the way we humans do, with smart individuals applying brainpower to puzzle them out.
I am not as forgiving. Dembski is not just an average joe expressing bewilderment at the “swarm intelligence” of ants. He is supposedly conversant with evolution and biology, and is making a pronouncement against evolution in a prominent place. He should have done his homework. Thanks to Alex for correcting him, and for demonstrating the unjustified eagerness of creationists like Dembski to say “evolution couldn’t have done that.”
